1. Field of the Invention
The invention relates to communication systems, and more particularly to an apparatus for achieving synchronization in a receiver.
2. Related Art
In synchronous digital transmission, information is conveyed by uniformly spaced pulses and the function of any receiver is to isolate these pulses as accurately as possible. However, due to the noisy nature of the transmission channel, the received signal undergoes changes during transmission and therefore, a complete estimation of certain reference parameters is necessary prior to data detection. The unknown parameters may cover such factors as the optimum sampling or the phase offset introduced in the channel or induced by the instabilities of the transmitter and receiver oscillators.
Estimation theory proposes various techniques for the estimation of these parameters; one such technique is Maximum Likelihood. However, ad-hoc techniques which are either totally unrelated to the optimum estimation derived from Maximum Likelihood or are at most loosely related, can also give excellent performance.
Generally, if the transmitter does not generate a pilot synchronization signal, the receiver must derive symbol timing from the received signal. The term symbol is used in this context to refer to transmitted signals that are phase modulated with discrete phase relationships. Both the transmitter and the receiver employ separate clocks which drift relative to each other, and any symbol synchronization technique must be able to track such drift.
Choosing the proper sampling instants is critical for reliable data detection. Failure to sample at the correct instants leads to Inter-Symbol Interference (ISI), which can be especially severe in sharply bandlimited signals. The term ISI refers to two or more symbols that are superimposed upon each other which makes phase detection of each symbol extremely difficult. Incorrect sampling implies the receiver is inadvertently sampling where the influence of the previous data symbol is still present (J. G. Proakis, xe2x80x9cDigital Communications,xe2x80x9d Third Edition, McGraw-Hill Publishers, pp. 536-537, 1995).
In a conventional digital receiver, the signal following demodulation is first passed through an anti-aliasing filter, which is used to limit the bandwidth of the received signal, and is subsequently sampled asynchronously. Asynchronous sampling implies there is no control over the instant at which the sampling of the continuous time signal occurs.
FIGS. 1-a and 1-b illustrate the concept of oversampling a continuous signal at four and two samples per symbol, respectively. The optimum sampling instants correspond to the maximum eye opening and are located approximately at the peaks of the signal pulses. FIG. 1-a shows that oversampling at four samples per symbol provides more information about the received signal than oversampling at two samples per symbol (FIG. 1-b).
The term xe2x80x9ceye openingxe2x80x9d refers to the amplitude variations of the signal at the output of the pulse-shaping filter. An xe2x80x9ceyexe2x80x9d pattern is formed by superimposing the output of the pulse-shaping filter for each symbol upon the other until the central portion takes on the shape of an xe2x80x9ceyexe2x80x9d. This is illustrated in FIGS. 2-a and 2-b for a BPSK (Binary Phase Shift Keying) modulation scheme for high and low signal to noise conditions. Note that at high signal to noise conditions the xe2x80x9ceyexe2x80x9d is open, whereas at low signal to noise conditions, the xe2x80x9ceyexe2x80x9d is closed.
Among synchronization techniques, a distinction is made between feedforward and feedback systems. A feedback system uses the signal available at the system output to update future parameter estimates. Feedforward systems process the received signal to generate the desired estimate without explicit use of the system output.
In an error tracking feedback loop, the timing estimator constantly adjusts the phase of a local clock oscillator to minimize the phase error between the estimated and the optimum.sampling instant as illustrated in FIG. 3. In a feedforward-timing loop, as illustrated in FIG. 4, the incoming signal is sampled asynchronously and applied to a timing estimator. This timing estimate is subsequently fed to an interpolator to estimate the received signal at the instant of the estimated timing offset phase. Interpolation estimates the signal value at the optimum sampling instant using the timing phase from the timing phase estimation unit (H. Meyr, M. Moeneclaey and S. A. Fechtel, xe2x80x9cDigital Communication Receivers: Synchronization, Channel Estimation and Signal Processing,xe2x80x9d John Wiley Publishers, Chapter 9, pp. 505-532, 1998). Redundant samples are removed using a decimator. Both feedforward and feedback techniques are currently in use; however, it should be noted that there are advantages and disadvantages associated with both approaches.
Problems with feedback techniques include the length of the acquisition time, the high probability of hangup and cycle slips associated with phase locked loop (PLL) based structures, especially in the presence of channel fading. Fading occurs when signal components arriving via different propagation paths add destructively. Hangup occurs when the initial phase error of the estimator is close to an unstable equilibrium point, which can result in an extremely long acquisition time (i.e., a long time for the loop to adjust to the correct phase); in fact, the loop may never recover. Hangup is very serious as it can even occur in perfect channel conditions (H. Meyr, M. Moeneclaey and S. A. Fechtel, xe2x80x9cDigital Communication Receivers: Synchronization, Channel Estimation and Signal Processing,xe2x80x9d John Wiley Publishers, pp. 94-97, 1998). Cycle slips are very harmful to the reliability of the receiver""s decisions, because a cycle slip corresponds to the repetition or omission of a channel symbol (H. Meyr, M. Moeneclaey and S. A. Fechtel, xe2x80x9cDigital Communication Receivers: Synchronization, Channel Estimation and Signal Processing,xe2x80x9d John Wiley Publishers, pp. 385-399, 1998).
These problems can be circumvented through the use of feedforward estimation. The advantages of feedforward estimation are that acquisition time is solely dependent on loop bandwidth and is not influenced by channel conditions. In addition, hangup does not occur and implementation costs are lower, as feedforward designs are more suited to VLSI (Very Large Scale Integration) implementation. Flexibility in the design of the synchronization unit has increased with the advent of increasingly powerful silicon chips.
However, feedforward techniques in the literature generally require a higher oversampling ratio than is prevalent in sampled feedback estimators (F. M. Gardner, xe2x80x9cA BPSK/QPSK Timing Error Detector for Sampled Receivers,xe2x80x9d IEEE Transactions on Communications, COM-44, pp. 399-406, March 1996), which generally require an oversampling rate of one or two samples per symbol for reliable operation. In feedforward designs, the oversampling rate is generally four or more samples per symbol (H. Meyr, M. Moeneclaey and S. A. Fechtel, xe2x80x9cDigital Communication Receivers: Synchronization, Channel Estimation and Signal Processing,xe2x80x9d John Wiley Publishers, pp. 289-295, 1998). This in turn is in contrast to analog feedback methods, which require a continuous waveform (J. G. Proakis, xe2x80x9cDigital Communications,xe2x80x9d Third Edition, McGraw-Hill Publishers, pp. 358-365, 1995). Digital synchronization methods recover timing by operating only on samples taken at a suitable rate. Digital implementation of an estimator has enormous appeal in communications technology and influences the design of all modem receivers.
There are two distinct stages involved in timing estimation: first, the estimation of the timing phase offset and second, the use of this estimate in the interpolationldecimation process. The estimated sampling instant within a symbol is the timing phase. The configuration of the feedforward timing estimator loop is very different from that of a feedback loop. FIG. 4 illustrates that, in a feedforward arrangement, sampling is asynchronous and the subsequent processing in the interpolator and decimator units must choose the optimum data samples for the received signal using an estimate of the timing offset based on the received signal samples. An algorithm is applied to estimate the timing phase in the timing phase estimation unit. What is desired is an improved timing phase estimation unit, wherein, for example, only two samples per symbol of the recovered signal are used to estimate the timing offset. Two forms of processing can be applied within the timing phase estimating unit depending on how the data present on the received signal is exploited to assist in the timing estimation. The first is data-aided (DA) estimation wherein known data within the data stream is exploited to improve the estimation performance. Alternatively, non-data-aided (NDA) estimation is possible wherein the arbitrary data is considered a nuisance parameter, which is removed by averaging the received signal over the statistics of the arbitrary data.
The received noisy signal samples contains no periodic components because the information symbols have zero mean. However, if the samples are passed through an appropriate nonlinearity, such as a square law operation, a cyclostationary process results which is observed as spectral lines in the frequency domain at multiples of the symbol rate. The phase of the spectral line at the symbol rate is related to the normalized timing offset. Exploitation of this and similar nonlinearities has been presented in the literature (M. Morelli, A. N. D""Andrea and U Mengali, xe2x80x9cFeedforward ML-Based Timing Estimation with PSK Signals,xe2x80x9d IEEE Communications Letters, 1(3): pp. 80-82, 1997; E. Panayirci and E. Y. Bar-Ness xe2x80x9cA New Approach for Evaluating the Performance of a Symbol Timing Recovery System Employing a General Type of Nonlinearity,xe2x80x9d IEEE Transactions on Communications; 44(1): pp. 29-33, 1996).
Timing estimators utilizing a nonlinearity are known as spectral line generating synchronizers. The expectation of the resulting signal at the output of the nonlinearity is a periodic signal with a period equal to the symbol rate T. This periodic signal can be composed as the sum of sinusoidal components (spectral lines) occurring at 1/T and multiples thereof. The signal at the nonlinearity output enters either a PLL or a narrowband bandpass filter. In the case of a narrowband bandpass filter tuned to the channel symbol rate 1/T, the sinusoidal component that is present at the output of the nonlinearity is isolated and serves as a clock signal for the sampling device.
An alternative to using a PLL or a narrowband bandpass filter involves evaluating the Fourier component of the spectral line occurring at the symbol rate. The accuracy of the estimate depends on the length of the observation interval over which the Fourier component is formed. The jitter on the estimate reduces as the observation interval increases. Exploitation of these and similar nonlinearities has been presented in the literature. However, these typically use a minimum of four samples per symbol. Therefore, there is a need for an estimator that has a feedforward configuration and can operate at a lower sampling rate.
If the timing estimator is among the first modules in a baseband receiver system, then the performance of the estimator should be independent of the presence of a phase offset on the sampled received signal which is deterministic or, at most, slowly varying over the observation interval.
Consequently, what is needed is to provide a timing estimation method that can give comparable performance in the presence of a slowly varying or static phase offset as alternative algorithms which require the removal of the phase offset beforehand. Timing estimation would be provided based on asynchronously sampling the pulse shaping filter output at a rate of two samples per symbol. The timing estimation would use a NDA technique and a feedforward configuration that avoids the problems associated with feedback design approaches such as hangup. The proposed estimator would employ two nonlinearities to generate spectral lines in the frequency domain with a period equal to the channel symbol rate. The phase of the spectral line generated at the symbol rate would be related to xcfx84/T, where xcfx84 is the timing offset. Two samples per symbol would provide sufficient information to isolate the timing offset as the argument of an expression formed using the outputs of the two nonlinearities. An embodiment of the estimator could then be used in conjunction with any of the established methods of timing correction available in the literature.
One aspect of the present invention includes a system and method of timing estimation for use in a digital receiver within a communication system. A timing estimation block or timing estimator is provided within a digital receiver where the inputted signal stream may be processed at two samples per symbol and the estimator operates in a feedforward manner. The invention implements an ad-hoc timing estimation technique whose performance is unaffected by the presence of a phase offset on the received signal samples. Furthermore, NDA estimation is utilized in the timing estimator.
This timing estimation method exploits the complementary information available from the use of two different nonlinearities to estimate the timing offset using two samples per symbol. The method uses a squaring nonlinearity as well as a delay, complex conjugation and multiply nonlinearity to generate the information to calculate the timing offset. Separating the two sample per symbol outputs from the two nonlinearities into odd and even samples yields four signal branches which, when suitably manipulated, give an expression for the timing estimate. This timing offset is then fed to a timing correction unit, which calculates the data samples corresponding to the sampling clock phase and removes the redundant samples. The resultant sampled signal is then forwarded to additional synchronization and functional units.
This method is provided for a variety of digital receivers employing Code Division Multiple Access (CDMA), in which a transmitted signal is spread over a band of frequencies much wider than the minimum bandwidth required to transmit the signal, Time Division Multiple Access (TDMA) where the users share the radio spectrum in the time domain, Frequency Division Multiple Access (FDMA) where a user is allocated at least one unique frequency for communication without interference with users in the same frequency spectrum, and/or any combination of the principles of the above or other technologies.
In accordance with another aspect of the invention, a digital receiver system comprises an anti-aliasing filter, a sampling unit, a filtering block, a timing estimation block, a timing correction block and additional functional blocks. The filtering block comprises a pulse-shaping filter. The timing estimation subsystem comprises a squaring nonlinearity, a delay, complex conjugation and multiply unit, two demultiplexers, two summation blocks, two subtractors and a phase calculator. The filtering block receives signals from an Intermediate Frequency (IF) block which have been demodulated to baseband. In one embodiment, these signals are sampled at the Analog to Digital Converter (ADC) with a fixed clock (Sampling Clock=46.7 MHz). It is necessary to note that the signals received by the filtering block within the digital receiver might not be sampled, and that the sampling may take place only after the filtering block. The outputted signals from the filtering block are then fed to the timing estimation subsystem for further processing. The resulting timing estimate is then fed to the timing correction unit to correctly estimate the received data stream. The output from the timing correction unit is subsequently fed to additional functional blocks for further processing.
In another aspect of the present invention there is a method of feedforward timing estimation for use in a digital receiver within a communication system, the method comprising splitting an input data stream into a first data stream and a second data stream; applying a first nonlinearity to the first data stream and a second nonlinearity to the second data stream; averaging the instantaneous values of the first nonlinearly derived data stream; averaging the instantaneous values of the second nonlinearly derived data stream; and determining a timing offset as a function of the complex components of the averaged instantaneous values.
In yet another aspect of the present invention there is a feedforward timing estimation system for use in a digital receiver within a communication system, the timing estimation system comprising a pulse-shaping filter receiving a data stream sampled at a minimum rate of two samples per symbol; a timing estimator, receiving the filtered data stream, and being capable of generating a timing offset; a processing delay, receiving the filtered data stream, and delaying the filtered data stream; and a timing correction subsystem, receiving the delayed data stream and the timing offset, and being capable of generating corrected data samples according to the timing offset.